How do the number of cores in the CPU relate to the rendering tiles?
Each thread of the CPU will render one tile. For example: if your computer's CPU has 4 cores and 8 threads, then blender will render 8 tiles simultaneously.

The number of threads used can be limited using a fixed number on the Performance section of the Properties Panel, the default is Auto-detect, which will use all of the available threads for rendering.

If you are using GPU, there will be one tile rendered per GPU. Multiple GPUs will allow you to render multiple tiles simultaneously.